1. Field of the Invention
The invention concerns a high temperature sensor, particularly an exhaust gas sensor in the exhaust line of an automobile.
2. Description of the Related Art
In order to meet the ever more stringent governmental requirements with respect to air quality, very selective gas sensors are necessary. Such sensors can be employed for example for monitoring pollutant levels, or to activate an alarm when a threshold concentration of a dangerous or poisonous gas in the environmental atmosphere has been exceeded. It is also possible to employ such gas sensors directly in the exhaust gas of an internal combustion process. Examples thereof include selective hydrocarbon sensors such as known for example from EP 0 426 989 or selective ammonia sensors as known for example from DE 197 03 796.
The mentioned examples concern gas sensors produced using planar technology (and in particular thick layer technology or thin layer technology). In FIG. 1 various views of the typical design of such a sensor are schematically illustrated. A substrate 4 has provided on the sensor lower side a structure 6 for heating and eventually temperature measurement, and has provided on the sensor upper side at the sensor tip a capacitor structure. This structure, which is again shown in FIG. 2 in enlarged representation, is comprised of a plurality of staggered or offset electrodes 8 which are alternatingly connected to conductor line 10 or conductor line 12. The conductors 10 and 12 have respective contact pads 14 and 16 on the sensor connection side, onto which connector wires are applied. If an alternating current is applied to the two conductors, then the capacitates CL of this structure (referred to in the following as empty capacity) can be measured. Since this capacitor structure looks similar to inter-digitating fingers, such a structure is referred to also as interdigitatec capacitor (IDC). If now upon this IDC structure a functional layer 18xe2x80x94not shown for purposes of better understandingxe2x80x94is applied, of which the electrical characteristic changes upon exposure to a gas, then one can construct therewith a gas sensor. Such a construction is in principle not only suitable for sensors which detect components of a gas mixture, but rather also for all chemical or substance sensors.
The term xe2x80x9csubstance sensorxe2x80x9d is intended herein to mean a sensor for determination of concentrations of a substance in a substance mixture, that is, for example, a sensor for determining the concentration of a component of a gas mixture or a sensor for determining a component of a fluid or a sensor which changes its output signal on the basis of an interaction with a gas or a fluid.
The above described arrangement comprised of substrate, heating and/or temperature measurement resistor device, and IDC structure will in the following be referred to as xe2x80x9cU-carrierxe2x80x9d. A sensor in this respect is also comprised of at least a transducer and a functional layer.
Estimation of Signal Size
The signal change to be measured depends upon the geometry of the IDC structure. This is shown again in FIG. 2 in enlarged view. The entire IDC structure has as external dimensions the length L and the breadth B. Across the breadth B electrode fingers of the breadth b are provided in separation s. One can therewith imagine the entire capacitor as a parallel circuit (electrically switched in parallel) comprised of multiple component capacitors, wherein each partial capacitor is comprised of two adjacent fingers. The empty capacity of these partial capacitors, and therewith also the total empty capacity CL, increases with the finger length L. With a reduction in the finger separation s the empty capacity of the partial capacitors likewise increases, since the density of the field line or line of electric flux between two fingers increases (in comparison: in plate capacitors the capacity is inversely proportional to plate separation). Since the total capacity is based upon the parallel circuitry of the partial capacities, the total capacity is the larger the greater the number of partial capacitors which can be provided within the breadth B with decreasing finger breadth b, thus the capacity of the total capacitor increases, since the number of the parallel switched partial capacitors increases with decreasing finger breadth b at constant outer dimension B. With decreasing finger spacing s in accordance therewith, the capacity of the total condenser even increases over-proportionally (almost quadratically), since on the one hand the number of the partial condensers and on the other hand their capacity increases.
The height of the electrode (layer thickness) is only of minimal consequence.
In the following, a few theoretical calculations of the total capacity CL will be presented, which are carried out using a finite element method. Therein, the measurements of a typical IDC structure, that is, approximately 5 mmxc3x976 mm (Lxc3x97B), is used as basis. For the relative dielectric constant, ∈r was presumed to have a value of ∈r≈10 as disclosed in published literature as conventional for Al2O3 substrates. The results of the calculations confirm that the layer thickness of the IDC structures can be disregarded.
It has further been determined, as best seen in FIG. 3, that an optimal relationship of line separation s and finger breadth b of s/b≈2 exists, at which the total empty capacity CL reaches a maximum. In FIG. 3, a finger separation of s=20 xcexcm was presumed. At a finger breadth of b=9.88 xcexcm, there is the maximum empty capacity. If one varies the finger separation s, then one can determine that the value of the optimal relationship is almost independent of the separation of the fingers. One achieves for example at s=20 xcexcm an optimal value for the finger breadth of b=9.88 xcexcm (s/b=2.024) and at s=10 xcexcm an optimal finger breadth of b=0.54 xcexcm (s/b=1.203).
The optimal empty capacity for finger separations ranging from 10 xcexcm to 30 xcexcm is shown in FIG. 4. One can recognize that at a finger separation of approximately 20 xcexcm, a total empty capacity CL of almost 40 pF can be achieved. Table 1 clearly shows the relationship between the geometric size b and s and the total empty capacity CL. At structure breadths for s and b of approximately 100 xcexcm, one achieves only a total empty capacity of CL less than 10 pF.
If one next applies the functional layer 18, then the measurable capacity increases, depending upon the dielectric constant ∈r of the functional layer and its thickness. It can however be shown that the influence of the layer thickness of the functional layer in particular at values of the dielectric constant ∈r less than 5 hardly plays any roll. If one presumes that the supplemental capacity, which is attributable to the functional layer, corresponds to the half value of the empty capacity, and if one further presumes that the supplemental capacity during gas sampling changes at a maximal of 10% of its value, then one obtains the maximal capacity change xcex94Cmax to be measured, which is entered in the fourth column of Table 1. It is immediately evident from Table 1 that one, in order to even be able to make reliable measurements, must have as small as possible finger breadth b and finger separation s. This is in particular then the case, when long conductors or lead lines, which conventionally exhibit capacities of a few pF/m, are required. This is for example the case, when the sensor is to be employed in the exhaust gas stream of an automobile, in order to be able to measure the ammonia or hydrocarbon content in the exhaust gas of an automobile. Therein, it is to be observed, that even this lead line or conductor capacity is conventionally not constant, but rather changes with the environmental temperature. This conductor capacity can only be compensated for in complex or expensive manner.
Further complicating matters is that small measurement currents are used. Thus, one calculates at an alternating voltage amplitude of 1 V and a capacity of 50 pF at a measurement frequency of 1 kHz a capacitive current of 314 nA, wherein the maximal signal change (that is, the measurement effect), however, only corresponds to approximately 16 nA. If one wants to resolve the sensor signal to 1%, then a measuring current of 160 pA must be resolved. Since the measuring current in a capacitive system with constant applied measurement voltage amplitude increases with increasing frequency, then one should measure at higher frequencies, which however may bring about a danger of intensified stray effect and electromagnetic interference. Since with a given measurement voltage the measurement current is proportional to the capacity, this is a further reason to select as fine as possible structures, that is, high capacity for the IDC structure.
The above discussed range of problems for functional layers, of which the capacitive characteristics change upon exposure to or interaction with gas, applies in appropriate manner also for sensors of which complex impedance (complex alternating current resistance) changes with gas sampling. Above all, high ohm functional layers, which provide only small capacitive values, require a fine as possible structure.
As a structure breadth which provides signals which are just barely detectable with economically justifiable measurement technology and subsequently electrically processable, 50 xcexcm has been found to be satisfactory.
Planar gas sensors can be produced either in accordance with the thick layer technique or the thin layer technique (typically processes of the thin layer technique: sputtering, vapor depositing, or CVD). Examples, in which also the processes are disclosed, which an be used for production of substance sensors in the thick layer technology, can be found in J. Gerblinger, M. Hausner, H. Meixner: Electric and Kinetic Properties of Screen-Printed Strontium Titanate Films at High Temperatures, J. Am. Cer. Soc., 78[6] 1451-1456 (1995) or M. Prudenziati (Editor): Thick Film Sensors, Particularly Section I: Thick Film Technology, pages 3-37, Elsevier-Verlag, 1994 or in DE 37 23 052. It is possible to combine thin layer techniques and thick layer techniques (so called hybrid technology), but this is expensive.
In the manufacturing of high temperature substance sensors, the following requirements are to be taken into consideration (the term high temperature sensors is understood to mean those sensors which are heated to temperatures above 300xc2x0 C. This type of requirement is placed particularly upon exhaust gas sensors, for example in the exhaust gas of internal combustion engines in vehicles):
On the one hand, thin layer techniques make it possible to produce the finest structure breadths of as small as a few xcexcm, which for the above mentioned example would be quite sufficient. However, thin layer processes only make possible layer thicknesses below 1 xcexcm. In rough to abrasive environmental conditions, in particular with long operation at high temperatures, such thin layers are not sufficiently durable over time. Further, it is necessary, when using conventional high temperature stable electrode materials, such as gold or platinum, for the thin layers, so called adhesion promoters which for example could be a few nm thick layers of chrome or titanium. At the high temperatures at which high temperature gas sensors operate, for example exhaust gas sensors, these materials diffuse to the upper surface of the electrode and there react with the functional layer 18. This changes the functional layer, and the sensor can become desensitized to the gas to be detected. Besides this many functional layers, in particular zeolites or complexes of multi-oxides cannot be produced in the thin layer technique. Besides this one requires for the production of components in the thin layer technology normally specific or particular substrates with a very low surface roughness, which is substantially more expensive (by a factor of five to ten) than conventional ceramic substrates. Since most thin layer processes are vacuum processes, one requires for the thin layer techniques complex and expensive apparatus, which can generally be amortized only when producing large patches of pieces.
The above discussed arguments lead to the conclusion, that the thick layer technique would be the most suitable manufacturing process for high temperature gas sensors both for technical as well as cost reasons.
However, unfortunately, using the thick layer technique conventionally, the finest structure breadths that can be reproducibly produced are only in the range of 70 xcexcm to 100 xcexcm. The required resolution of below 50 xcexcm, in particular approximately 20 xcexcm, could not be achieved with the conventional thick layer techniques for gas sensors according to the state of the art.
It is thus the task of the invention to provide a high temperature substance sensor with structural sizes smaller than 50 xcexcm, with which the described range of problems with respect to the manufacture of the sensor can be overcome.
In accordance with the invention, the production of the capacitor or capacitor structure of the high temperature substance sensor occurs from a combination of the thick layer technique process and a photolithographic structuring process, which is employed in the planar technique for production of semiconductor components. It is now for the first time employed in the manufacture of substance sensors. The production of the other layers of the high temperature substance sensors occurs advantageously using the thick layer technique, for example, with the silkscreen printing or stencil printing technique.
For the production of the capacitor structure, there is first produced, using the thick layer technique, a complete (closed) or already pre-structured capacitor layer as a precursor of the capacitor structure. Subsequently, there occurs the structuring of the capacitor layer using photolithography.
The inventive high temperature substance sensor is particularly suitable for employment has exhaust gas sensor in internal combustion exhausts, for example, in the exhaust of an automobile.
It can be constructed for example as an ammonia or hydrocarbon sensor.